For example, conventional optical coupling systems between LD (semiconductor laser) and an optical fiber to be used in an optical module are shown in FIGS. 1A and 1B. The first system is so composed that a LD element 1 and an optical fiber 2 are, as shown in FIG. 1A, directly optically coupled. The second system is so composed that a lens 3 is, as shown in FIG. 1B, inserted between the LD element 1 and the optical fiber 2, whereby light beams to be emitted from the LD element 1 are converged and coupled to the optical fiber 2.
Furthermore, there is another system that liquid 4 with a proper refractive index as a refractive-index controlling material is, as shown in FIG. 1C. injected to relax the coupling tolerance. In this system, the process of injecting liquid 4 into the optical module and the airtight sealing process are complicated. Therefore, the productivity must be lowered.
Meanwhile, the conventional airtight sealing structure of the optical module is given by mounting an optical coupling system in a package (PKG) made of metal or ceramic, thereafter airtightly sealing a metal or ceramic cap by using seam welding, glass seal, solder seal etc.
However, the conventional coupling systems between the LD element and the optical fiber have several problems. First, the direct coupling system shown in FIG. 1A, which can be cheaply fabricated as having the simple structure, has a low coupling efficiency between the LD element 1 and the optical fiber 2 since the spot sizes of the LD element 1 and the optical fiber 2 are different from each other, so that the optical power of the optical module cannot be increased.
Second, the optical coupling system using the lens 3 as shown in FIG. 1B, in which the coupling efficiency can be enhanced since the beams emitted from the LD element 1 are converged, needs to fix the three components, i.e., LD element 1, lens 3 and optical fiber 2 when fabricating the optical module, thereby complicating the fabrication. Further, the optical axis needs to be regulated precisely. Thus, the productivity of the optical module must be lowered.
Third, in the method of injecting liquid 4 with a proper refractive index as a refractive-index controlling material as shown in FIG. 1C, the process of injecting liquid 4 into the optical module and the airtight sealing process are complicated. Therefore, the productivity must be lowered.
Furthermore, in the conventional airtight sealing techniques, there are problems that the shape of the optical module is limited depending on the respective sealing techniques and a member to be used inside the module needs to be heat-proof as it is heated at a high temperature in the airtight sealing process.